hyphenation-character doesn't appear in <programlisting>

I tried to change the hyphenation-character with a backslash in <programlisting> tag but it is not
appearing in the pdf output. However, long line in pdf are broken down to multiple lines.
Can some1 tell me what is wrong with my customization layer?
I'm using Docbook 1.73.2 and FOP v0.94.

Here is my customization layer:
    <xsl:param name="hyphenate.verbatim" select="1"></xsl:param>
    
    <xsl:attribute-set name="monospace.verbatim.properties"
use-attribute-sets="verbatim.properties monospace.properties">
      <xsl:attribute name="wrap-option">wrap</xsl:attribute>
      <xsl:attribute name="hyphenation-character">\</xsl:attribute>
    </xsl:attribute-set>

thx!



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